1. Field of the Invention
The present invention relates to a drive circuit for at least one light-emitting diode by means of which the brightness of the light-emitting diodes (LEDs for short) is regulable to a setpoint value.
2. Description of the Related Art
Various such drive circuits are known from the prior art. Thus, for example, U.S. Pat. No. 6,016,038 discloses a system for driving LED arrangements for illumination purposes. As shown, in particular, in FIGS. 1 and 2 of the U.S. Patent, the individual LEDs or LED groups are driven by a constantly regulated maximum current flowing through the LEDs. The constant maximum current is regulated by means of integrated circuits. The maximum current regulated in this way is switched on and off by means of a current sync, wherein the current sync can be driven in accordance with the desired ratio of the off times and on times of the current. The brightness generated for the observer of the LEDs or LED groups can thus be determined, as is known, by the ratio of the on and off times of the current through the LEDs.
The object of the present invention is to provide a drive circuit for at least one light-emitting diode with which the brightness of the light-emitting diodes can be regulated in a low-loss way and independently of possible voltage fluctuations in the voltage supply.
This object is achieved by a drive circuit which has a switched-mode regulator, a means for determining current through a light-emitting diode connected to the drive circuit, a regulating device and a controller. The switched mode regulator is constructed so that it can be supplied with a direct input voltage to drive a light-emitting diode. The means for determining current through the light-emitting diode is also constructed to determine the voltage across the diode. The regulating device is configured so as to be fed with a current setpoint value and a voltage setpoint value and with signals representative of the determined current and the determined voltage at the light-emitting diode. The regulating device is also constructed to regulate the current and the voltage supplied to the light-emitting diode according to the respective setpoint values; and the controller is arranged for adjusting the setpoint values.
In the drive circuit according to the invention, the current and/or the voltage of the LED(s) is regulated to an adjustable setpoint value, the setpoint value being adjustable by means of a controller. The LEDs are consequently dimmed directly through the dimming of the current and/or the voltage.
Preferably, the switched-mode regulator can be integrated into an AC/DC converter that supplies the circuit with the necessary direct voltage. That is to say, the AC/DC converter may be designed in such a way that it regulates directly to the desired current value or voltage value.
In a preferred embodiment of the invention, the controller alters the switching behavior of the switched-mode regulator driving the light-emitting diodes by adjusting the setpoint value in such a way that the switching frequency or the duty cycle of the switched-mode regulator is altered. Preferably, the current or the voltage is determined by means of a measuring shunt connected in series or parallel with the light-emitting diode to regulate the current or the voltage, respectively, and/or determines the brightness of the light-emitting diodes by means of a photodiode disposed in the immediate neighborhood of the light-emitting diodes.
In another preferred embodiment of the present invention, the controller alters the resistance value of a variable resistor connected in series with the light-emitting diode to adjust the setpoint value of the current through the light-emitting diodes. In this embodiment, the current through the light-emitting diodes is determined by means of the voltage dropped across the variable resistor and/or the brightness of the light-emitting diodes is determined by means of a photodiode disposed in the immediate neighborhood of the light-emitting diodes.
Further preferred embodiments and developments of the invention are described and claimed herein.
Another development of the invention relates to measures which enable the dimensions of the drive circuit to be kept as compact as possible. In order to achieve this, the drive circuit comprises, at least partly, a multilayer circuit into which passive components, for example resistors, conductor tracks and the like, are integrated. This integration is possible, in particular, if the switched-mode regulator is operated at high frequencies since correspondingly lower capacitance values or inductance values can then be used in the circuit. In the present case, a frequency range of 200 kHz to 1 MHz has proved to be particularly suitable. An increased radiation of electromagnetic high-frequency fields initially due to the increase in frequency can be avoided by suitable screening measures that can easily be undertaken (because of the reduced dimensions of the circuit).
Components can be integrated, for example, by means of multilayer printed circuit board technology. Preferably, the multilayer circuit is implemented by an LTCC (low temperature co-fired ceramic) structure that comprises a plurality of low-sintering ceramic layers or sheets that are disposed above one another and between which conductor tracks are situated. Compared with conventional printed circuit board technology, said LTCC technology, which has been newly developed in recent years and is disclosed, for example, in EP 0 581 206 A2, can achieve a further miniaturization of the circuit. In this technology, inductances and capacitances, in particular, can also be integrated into the multilayer circuit in addition to the conductor tracks. Furthermore, the ceramic material offers the advantage that it conducts heat relatively well, which means that, for the same overall volume, greater powers can be achieved since heat loss is radiated better. Preferably, the heat dissipation is increased yet again by encapsulating the ceramic structure in a metallic housing. An effective screening of the high-frequency fields radiated into the environment by the drive circuit can also be achieved in this way.
At the abovementioned frequencies, many of the components of the drive circuit can be integrated into the multilayer circuit. The remaining passive components and semiconductor chips have, however, still to be mounted on the surface or outside the ceramic structure. In order to achieve as low a space requirement as possible for this purpose, the semiconductor chips are preferably mounted by means of the known flip-chip (FC) technology on the ceramic substrate. In this case, there is inserted between the semiconductor mounted without housing and the contacts on the surface of the carrier substrate a plastic layer that is, on the one hand, conductive perpendicularly to the connection level and is insulating in the connection level and that, on the other hand, absorbs the stresses occurring in the case of different thermal expansion of the semiconductor chip and of the ceramic substrate and, consequently, prevents destruction of the semiconductor chip.
The invention is explained in greater detail below using various preferred exemplary embodiments with reference to the accompanying drawings.
FIG. 1 is a block and circuit diagram of a circuit which constitutes a first exemplary embodiment of a drive circuit according to the invention;
FIGS. 2 to 7 are views similar to FIG. 1 but showing circuits which constitute the diagrammatic circuit design of further exemplary embodiments of a drive circuit according to the invention;
FIGS. 8a and 8b show the plot, against time, of control signals and an LED current in the exemplary embodiments of FIGS. 6 and 7;
FIG. 9 is a block and circuit diagram of a circuit which constitutes another exemplary embodiment of a drive circuit according to the invention;
FIGS. 10a-15 are plan and cross-sectional views illustrating steps in the manufacture a multilayer ceramic (LTCC) structure according to the invention; and
FIG. 16 is an enlarged cross-sectional view showing a portion of an LTCC structure according to the invention.